1. Field of the Invention
The present invention relates to the shaping of glass sheets, particularly exceptionally large sheets having a non-rectangular outline shape. The glass sheets of the present invention are suspended by tongs and supported thereby for movement through a tunnel-type furnace having a vertical dimension only slightly larger than the vertical dimension of the glass sheet in the orientation suspended from the tongs. The nature of the bend imparted to the glass is such that the lower edge of the glass sheet requires additional heat after the glass sheet leaves the tunnel-type furnace to insure that the shaping of the glass does not cause the glass sheet to develop fissures that lead to breakage.
2. Technical Background and Patents of Interest
Glass sheets are usually shaped successively in a mass production operation by heat softening the glass sheets while conveyed in succession through a tunnel-type furnace. Thereafter, the leading glass sheet of the series moving through the furnace is removed from the furnace. While the removed sheet is still sufficiently hot to be deformed, a pair of press bending molds of complementary curvature sandwich the heat softened glass sheet in pressurized engagement. This pressurized engagement shapes the glass to a desired curvature. After shaping, the bent glass sheet may be quenched, if desired, before it cools to a temperature below that suitable for imparting a temper in the glass by rapid cooling.
If it were practical to maintain the press bending molds and their actuating means within the furnace, the glass sheet would be shaped while within the furnace. However, the elevated temperature of the furnace is not conducive to durability of the moving parts that energize the movement of the press bending molds toward and away from one another. Furthermore, the press bending molds of the glass shaping art are usually provided with covers of a suitable fibrous material such as knit fiber glass cloth to prevent direct contact between the surfaces of the press bending molds and the heat softened glass surface. The fiber glass cover insulates the glass from direct contact with the mold and prevents undesirable marks in the glass surface replicating any departures from surface smoothness in the mold. Furthermore, if the mold were located within the furnace to provide a press bending operation, the fiber glass covers would become worn quite rapidly and thus would require frequent replacement. It would be impossible to replace the fiber glass covers within the furnace without discontinuing the furnace operation to enable operators to obtain access to the molds to remove the covers and replace them with new ones. For reasons just explained, a practical operation requires that a glass shaping station be located outside the furnace.
It has been suggested to heat the press bending molds to elevated temperatures outside the furnace. However, such heating is economically impractical because the large mass and high heat capacity of the press bending molds would require much thermal input to obtain the desired mold temperature. Furthermore, the fiber glass covers would require frequent replacement because they would be continuously exposed to the high temperature imparted to the press bending molds and also would be subject at these high temperatures to the friction involved in sandwiching the covers between moving press bending molds and the opposite surfaces of a heat softened glass sheet.
For reasons just explained, glass sheets have been shaped by pressurized contact with relatively cold press bending molds located outside a tunnel-type furnace. During this operation, and particularly during the shaping, the glass sheet is supported in a substantially vertical orientation, preferably by glass gripping tongs which engage a relatively small portion of the glass sheet and interfere to a minimum with the heating and subsequent cooling of the glass sheet subjected to shaping and quenching operations. However, the present invention to be described later may be employed in conjunction with any well known technique for supporting a glass sheet in a vertical, near vertical or oblique orientation. Several of these alternate glass sheet supporting techniques are described in U.S. Pat. No. 3,333,934 to Seymour, the description of which is incorporated herein by reference.
In any shaping operation wherein the glass is supported in a vertical or oblique orientation, the temperature to which the glass is heated is very critical. If the glass is heated to too high a temperature within the furnace, the glass sheet becomes misshapen and distorted before it reaches a position between the press bending molds. If the glass sheet is not heated to a sufficiently high temperature, it may be chill cracked during the press bending operation.
When a vertically suspended, hot glass sheet of uniform temperature from top to bottom leaves an enclosed furnace, a vertical temperature gradient tends to be established with the lowest temperature along the bottom edge of the glass sheet. This temperature gradient tends to result in temporary tension stresses in the bottom portion of the glass sheet during its press bending. The magnitude of the tension stresses is a function of the steepness of the temperature gradient in the lower portion of the glass sheet. Recently, there has been a need for bent, tempered glass sheets having a vertical dimension longer than those fabricated in the past because of the need for fabricating large tempered glass sheets for automobile rear windows. The fabrication of these larger glass sheets caused a problem of venting along the bottom edge because when the exit door of the furnace opened, a rush of cold ambient air into the furnace from outside the furnace was closer to the bottom portion of the glass sheet than previously fabricated glass sheets of lesser vertical dimension suspended from tongs. This caused the distance between the bottom edge of the glass sheet and the region of coldest air flow into the furnace to be relatively short when fabricating bent glass sheets of larger vertical dimension. Consequently, the temperature gradient from top to bottom of the larger glass sheets leaving the furnace was more severe than was the case with glass sheets of shorter vertical dimension previously fabricated. As a result, the incidence of breakage along the bottom edge of longer (i.e., higher) glass sheets was greater than for smaller glass sheets produced earlier.
The tension stress resulting from the temperature gradient developed when a longer glass sheet left the furnace was often too large for the glass sheet to resist spontaneous fracture caused by the unbalanced stress resulting from the glass sheets cooling from top to bottom from vastly different furnace exit temperatures during their transit to the shaping station and from unbalanced stresses induced by contact of the major surfaces of the differentially cooled sheets with the press bending molds.
In U.S. Pat. No. 3,333,934 to Samuel L. Seymour, the bottom edge portion of a glass sheet is exposed to additional selective heating after its removal from the furnace en route to a shaping station. This selective heating is accomplished by directing burning gas in an upward direction from a straight, horizontal, apertured pipe against the lower portion of the glass sheet. The burning gas flows upward over the major surfaces of the glass sheet in such a manner that the heating effect of the rising gas flames is greatest at the lower edge of the glass and diminishes in an upward direction. Since heated glass sheets cool outside the furnace more rapidly at their bottom portion while the rising heat tends to impart some heat to the upper portion of the glass sheet, the upward flow of the gas flames along the glass surface outside the furnace tends to compensate for the natural rate of cooling by providing the greatest concentration of heat where the rate of natural cooling is greatest and a lesser concentration of heat where less heat is required to compensate for the natural cooling effect.
The Seymour patent discloses an elongated pipe having a plurality of closely spaced apertures aligned with and facing upward toward the path of movement taken by the glass sheets. Means is provided for supplying gas under pressure to the elongated apertured pipe for burning at the outlets of the aligned apertures to provide a vertical wall of flame intersecting the horizontally extending path of movement of the glass sheets into the shaping station. The wall of flame so developed originates slightly below the bottom edge of the glass and is directed against the bottom edge of the glass.
The additional heating that the glass sheet undergoes outside the furnace immediately before its shaping enables the glass sheet and particularly its bottom edge portion, to be at a temperature sufficient for shaping without requiring overheating of the glass within the furnace. The additional heating beyond the furnace compensates for the heat loss that takes place while the glass is transferred from the furnace to the shaping station.
The Seymour patent discloses two embodiments. In one, the gas pipe is located at the glass shaping station below and between a pair of press bending molds when the latter are retracted. Since the gas pipe is rigidly placed in position, this embodiment has certain drawbacks in that the heat of the flames causes rapid wear of the press bending molds, particularly their fiber glass covers, when the molds engage the glass sheets.
In a second embodiment of the patented Seymour invention, the shaping station is separated from the exit door of the furnace by a considerable distance and an elongated apertured pipe is rigidly supported between the exit door of the furnace and the glass shaping station. The elongated apertured pipe extends horizontally in longitudinal alignment with the path of movement taken by the glass sheets traversing the path between the furnace and the shaping station. The pipe in this second embodiment terminates short of the shaping station. The gas burner of the second embodiment does not apply the gas flames directly to the covers of the press bending mold. Nevertheless, the bottom edge of the glass is exposed to the ambient atmosphere after it traverses the length of the pipe between the furnace exit and the shaping station. Under such circumstances, the overall heat imparted to the glass sheet within the furnace radiates to cool the glass sheet. Consequently, it is difficult to have the glass sheet maintain a temperature sufficient for shaping at the press bending station using this embodiment. Furthermore, even though the bottom edge of the glass has been heated selectively during its passage toward the shaping station, the bottom edge still has a chance to cool to a temperature at which some chill cracking associated with the establishment of a temporary tension stress can be experienced.
In addition, both embodiments of the Seymour invention, which incorporate a stationary pipe, disclose a straight pipe that extends horizontally. A straight pipe provides clearance for moving a glass sheet, but accomplishes substantially uniform bottom edge heating only for a glass sheet having a straight horizontally extending bottom edge. The need to fabricate bent glass sheets having an edge curved in outline means that the straight pipes needed to insure no interference with the movement of the glass sheets along a path from the furnace to the shaping station and beyond to a cooling station if the latter is needed, would cause non-uniform heating along the length of the bottom edge. This non-uniform heating might induce non-uniform temper stresses in the direction of movement.
Another patent that involves the press bending of glass sheets with auxiliary gas burners is U.S. Pat. No. 3,333,935 to Clement E. Valchar and Stanley J. Mrozinski. In this patent, the auxiliary gas burners are located in position to apply local heat selectively to regions of a glass sheet to be bent sharply to insure rapid flow of the glass regions from a flat to a sharply curved configuration when the glass is bent to a shape involving a non-uniform radius of curvature. In this patent, portions of the covers for the press bending molds are damaged by the localized high heat and require frequent replacement.
At the time of the present invention, the art of shaping large glass sheets by press bending required auxiliary heating means available for use outside the furnace and in closely spaced relation to the furnace and in closely spaced relation to the position occupied by the glass sheet at the press bending station. Such auxiliary heating means were needed to provide a substantially uniform heat along the bottom edge portion of the glass sheet during the time that the glass sheet entered the shaping station. Such auxiliary heating means was also required to avoid harming the covers of the press bending molds when the latter move to close the gap therebetween.